This disclosure relates to trapping to reduce the effects of printer misregistration.
The problem of misregistration is a mechanical problem, almost always existing in printing systems. The problem arises because color separations are not laid exactly where intended, due to inherent imperfections in any separation registration system. It is somewhat correctable by mechanical registration methods; however it is rarely completely correctable. In expensive, high-end printing processes, customers have high expectations that misregistration artifacts will not be visible. In inexpensive, low-end printers, mechanical registration techniques are so expensive as to be infeasible; thus, a non-mechanical method of correction, often referred to as trapping, is typically used instead.
Different printing technologies have distinct misregistration artifacts. Offset printing tends to have uniform misregistration in all directions. However, xerographic printing tends to have more misregistration in a single direction.
Methods for correcting misregistration are known. The general approach is to expand one of the abutting regions' separations to fill the gap or misregistration border region with a color determined to minimize the visual effect when printed. Borders or edges expanded from a region of one color to another in this manner are said to be “spread.” A border, which has been expanded, is referred to as a “trap,” and the zone within which color is added is called the “trap zone.”
Commonly used methods for automatic trapping of digital images fall into the categories of vector-based and raster-based methods. Vector-based methods rely on images that have been converted from a page-description language form, describing objects as characters, polygonal shapes, etc. into an internal data structure containing not only object information, but also a list of all the edges between regions of different color. Raster-based methods rely on images that have been first scanned or converted from page-description based form and are stored internally as a sequence of (high resolution) scan lines each containing individual scan elements or pixels. These methods process each raster line in sequence and compare one or more adjacent pixels to determine color boundaries. After some initial processing to find edges, both vector-based and raster-based methods apply rules for determining whether or not to create a trap at such boundaries, and finally apply a second set of rules to determine the nature of the trap if one is to be created.
Now that trapping has been introduced, a specific type of printer will be discussed. A significant amount of color printing can be performed using a highlight color printer. In this type of printing, two inks are typically used in the printing process. These inks typically comprise black and a highlight color (specific examples are red or blue). Electronic printers may be designed specifically for highlight color printing. Highlight color printers are generally faster and less expensive than full color printers because only two inks are processed, as opposed to the three or four inks, which can be processed in order to obtain full color images. One such application is a teacher's edition textbook, which includes single color main text, such as black, and highlight color answers, printed in a highlight color such as red. For a highlight printer, besides black toner, additional color toner is used to add the accent in the document to draw attention. Other examples of accented areas of a document include a company logo, the balance in the bill, an important message in a document, etc.
Occasionally, the region covered only by the black toner and the region covered only by the highlight toner abut each other. If the registration is off and these regions are pulled away from each other, one would observe a white gap between the regions exposing an uncovered print medium.
For exemplary purposes, FIGS. 1 and 2 show an image with perfect registration. FIG. 1 shows a black region 10 and a black region 20 are disposed in, but do not overlap with, a block of highlight color 30. FIG. 2 is an enlargement of FIG. 1. Hereinafter, the specification will describe this block of highlight color 30 as halftone, but it is understood that this block of highlight color 30 may be solid.
In contrast, FIGS. 3 and 4 show the same image of FIG. 1 printed with misregistration. In this case, the highlight block 30 is shifted up and to the left relative to black regions 10, 20. FIG. 3 shows how the image under misregistration causes the white gaps 40, 60 to appear because of the exposed white print medium. FIG. 3 also shows how the misregistration creates an overlapped area 50 (from the highlight and black bitmaps). The white gaps 40, 60 and overlapped area 50 are artifacts of the misregistration. FIG. 4 is an enlargement of FIG. 3.
Some conventional trapping methods are disclosed in United States Patent Application Publication 2008/0239343 and U.S. Pat. No. 7,139,098, both of which are incorporated herein by reference in their entirety.